fixed migration example in guide

do not register the `app\migrations` path twice when using namespaces.

fixes #14536

docs/guide/db-migrations.md

@@ -909,6 +909,7 @@ return [
     'controllerMap' => [
         'migrate' => [
             'class' => 'yii\console\controllers\MigrateController',
+            'migrationPath' => null, // disable non-namespaced migrations if app\migrations is listed below
             'migrationNamespaces' => [
                 'app\migrations', // Common migrations for the whole application
                 'module\migrations', // Migrations for the specific project's module